Encoders are used to measure angular or linear displacement. Optical encoders include light sensor arrays, e.g., photodiode arrays, arranged in a detection plane for detecting incremental signals indicative of relative movement, e.g., rotary or linear movement, of encoder parts. Optical encoders include a light source, a scale index member (such as, e.g., a slotted or graduated code disk), and an array of photodiodes. A graduated code disk includes alternating transparent and opaque areas arranged periodically in a defined, fixed code disk pitch. In rotary encoders, the code disk is typically coupled to a rotating shaft and moved relative to the light source and the light sensor array.
Light is directed from the light source to the light sensor array by the scale index member or code disk. As the code disk rotates in the case of an angular encoder, or moves linearly in the case of a linear encoder, output signals from the light sensor array change. These varying output signals are used to measure angular or linear displacement.
Rotary encoders may be used in connection with electric motors, such as brushed and brushless motors. Brushed and brushless motors are driven differently, and a rotary encoder may play a more involved role in the driving of brushless motors.
A brushed direct-current motor applies current to a stator, which is typically fixed with respect to the motor housing, and a rotor, which rotates with respect to the stator. In this typical arrangement, the rotor acts as an electromagnet, whereas the stator has stationary permanent magnets. The force needed to drive the rotor within the stator is provided by the attraction and repulsion between the electromagnet in the spinning rotor and the permanent magnets in the stator. This arrangement typically requires changing the polarity of the electromagnet of the stator as the stator performs each full rotation. This changing of polarity is accomplished with a brush arrangement whereby two or more brushes, which are typically fixed with respect to the housing, alternatingly contact two or more electrodes of the rotor to provide electrical power to the electromagnet. The alternating contact is provided due to the fixed placement of the electrodes on the rotor. Thus, the rotation of the rotor with respect to the brushes causes each electrode to alternatingly physically contact each of the two brushes. In this manner, the polarity of the electromagnet of the rotor is alternated. The rotary switching brush mechanism is considered a mechanical commutator.
Brushless motors, which dispense with the brush arrangement, provide one or more electromagnets on the stator, while the rotor is provided with an arrangement of permanent magnets. In this arrangement, a direct current to the electromagnets of the stator is switched to actuate the rotation of the rotor. Since a brush mechanism, i.e., a mechanical commutator, is not provided, brushless motors typically utilize an electronic commutation system to control the respective polarities of the electromagnets of the stator.
To facilitate the electronic commutation, it is advantageous to determine the rotational position of rotor with respect to the stator. Based on the determined position, commutation circuitry controls the steering of current through the windings of the electromagnets of the stator.
One mechanism for determining the position of the rotor for commutation is to provide a circuit board having Hall-effect devices. This mechanism provides Hall-effect sensors on the circuit board. The circuit board is aligned with the rotor such that one or more magnets on the rotor communicate with the Hall-effect sensors during rotation of the rotor.
For applications where positional feedback is desired, e.g., in servo applications, the Hall-effect switching can add bulk to the motor system, since the Hall-effect circuit board is provided for commutation, while an encoder is provided separately for precise positional feedback. In these and other applications, it may be advantageous to utilize the encoder code disk for the commutation instead of a Hall-effect arrangement. Using an optical encoder, for example, the code disk may be provided with a plurality of commutation tracks that allow light to be blocked or passed from the light source to the photodiode array depending on the position of the rotor with respect to the stator.
Where the commutation tracks are provided as cutouts in the code disk, the mechanical integrity of the code disk may be substantially compromised due to the removal of material in the relatively thin metallic disk. This may be especially problematic where the brushless motor has a relatively small number of poles, e.g., two, which typically leads to longer commutation tracks and, hence, longer continuous cutouts of code disk material. For example, in a two-pole motor, each of three commutation track slots would need to extend approximately 180 mechanical degrees around the center of the code disk. As such, where low pole-number motors are desired, e.g., in higher speed applications, Hall-effect commutation sensors are generally used for commutation.
In addition to commutation tracks, other code disk tracks or slots may present analogous problems when they are sufficiently elongated along the code disk.